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| Titel |
Molecular multiproxy analysis of ancient root systems suggests strong alteration of deep subsoil organic matter by rhizomicrobial activity |
| VerfasserIn |
Martina Gocke, Arnaud Huguet, Sylvie Derenne, Steffen Kolb, Guido L. B. Wiesenberg |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250083301
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| Zusammenfassung |
| Roots have a high potential capacity to store large amounts of CO2 in the subsoil. However,
associated with rooting, microorganisms enter the subsoil and might contribute to priming
effects of carbon mineralisation in the microbial hotspot rhizosphere. Although these
processes are well known for recent surface soils, it remains questionable, if and how
microorganisms contribute to priming effects in the subsoil and if these effects can be traced
after the roots‘ lifetime. The current study implies several state-of-the-art techniques like
DNA and lipid molecular proxies to trace remains of microbial biomass in ancient root
systems. These can provide valuable information if parts of the root and rhizomicrobial
biomass are preserved, e.g. by encrustation with secondary carbonate during the root’s
lifespan or shortly thereafter.
At the Late Pleistocene loess-paleosol sequence near Nussloch (SW Germany), rhizoliths
(calcified roots) occur highly abundant in the deep subsoil from 1 to 9 m depth and below.
They were formed by Holocene woody vegetation. Their size can account for up to several
cm in diameter and up to > 1 m length. Rhizoliths and surrounding sediment with increasing
distances of up to 10 cm, as well as reference loess without visible root remains
were collected at several depth intervals. Samples were analysed for n-fatty acids
(FAs) and glycerol dialkyl glycerol tetraethers (GDGTs; membrane lipids from
Archaea and some Bacteria), as well as structural diversity based on the RNA gene
of the prokaryotic ribosome subunit 16S (16S rRNA). GDGT represent organic
remains from microbial biomass, whereas FA comprise both microbial remains and
degradation products. 16S rRNA indicates the presence of both living cells and/or cell
fragments.
Despite the general low RNA contents in the sample set, results pointed to a much higher
abundance of bacterial compared to archaeal RNA. The latter occured in notable amounts
only in some rhizoliths. This was in part enforced by decreasing contents of archeal
GDGTs from rhizolith via rhizosphere towards root-free loess. Furthermore, the
bacterial fingerprint revealed – similar to modern root systems – higher taxonomic
diversity in rhizosphere compared to rhizoliths and reference loess. This argues for
microorganisms benefiting from root deposits and exudates. Highest concentrations of
branched GDGTs in rhizoliths suggest that their source organisms feed on root
remains. Incorporation of rhizomicrobial remains as represented by RNA and GDGTs
usually affected the sediment at maximum to a distance of 2-3 cm from the former
root.
FA contents in rhizosphere showed strong scatter and were in part depleted compared to
reference loess or, especially in deeper transects, enriched. This indicates the presence of
degradation products originating from former rhizosphere processes. Especially at larger
depth not affected by modern pedogenic processes, portions of mainly microbial
derived C16 homologues were higher in rhizosphere loess up to distances of 10
cm, revealing that the possible extension of the rhizosphere was underestimated so
far.
In Corg poor subsoil, the occurence of diverse rhizosphere microorganisms and
degradation processes even in several centimeters distant from roots point to a strong
alteration of OM, possibly contributing to carbon mineralisation. |
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